Add groups and group-based tagging

[noah-schechter]

It sounds like it'd be helpful for folks to be able to ping a group of people in a comment. I think longer-term we should turn flair into a deeper part of the data model—it should be a sort of group that someone can be added to by a project owner. Anyone with a specific flair should be able to be tagged in a comment. If I'm a member of ABCD investigations, and my username is @noah ABCD Staff, an @noah, @ABCD, @Staff, or @all would notify me.

Potential problems:

• Folks would have different flair in each project. Maybe this is fine?
• New users couldn't choose usernames that match any actively-used flair. Flair would also be limited by actively-used usernames.

[noah-schechter]

Shorter-term, we could add @all and restrict this as a username.